Methods are known for the attenuation of the noise produced by gas jets by active silencers, or the isolation and reflection of the sound waves to their source or in some other direction by reactive silencers.
The active silencers have the shape of conduits whose inner surfaces are lined with sound-absorbing material; they are built in various versions: with simple chambers, with lamellar or cellular elements and with chambers and screens.
The simple chamber silencer consists of a tube made out of steel plate or of some other material to which the sound-absorbent treatment (sound-absorption layer) is applied only at the walls.
To increase sound-absorbing capacity over a broader frequency range, large-section conduits are divided into a series of subconduits with reduced dimensions by means of sound-absorbing plates parallel to the flow direction and arranged in line with one or both axes of the conduit section (the lamellar or cellular silencer).
The chamber and screen silencer consists of one or several chambers acoustically lined and separated by means of screens arranged normally or obliquely to the path of gas flow. Noise attenuation is achieved by arresting the energy of the sound waves by means of the sound absorbent treatment in order to reflect them to the screens.
The reactive silencer is an acoustic system which allows passage practically without attenuation of sounds of a certain frequency while damping or reflecting towards the source sounds of the remaining frequencies. This acoustic system consists of several chambers successively joined to one another by tubes. Each chamber with its junction constitutes a resonator which damps within a certain frequency range.
It has also been proposed to effect noise attenuation by increasing the surface of the gas stream and ambient air mixture with multisection silencers. These silencers achieve the partial attenuation of the noise by increasing the surface of the gas stream and ambient air mixture while avoiding any turbulence. Under these circumstances quick diminution of the gas velocity is attained along the jet axis and implicitly the attenuation of the low-frequency noises.
There are likewise known nondirectional silencers used especially for the attenuation of the noise generated by the exhausts of internal combustion engines. These silencers have spral or baffle conduits which are provided with orifices or groups of orifices arranged in certain arrays and sometimes accompanied by deflecting cups covering the orifices. Noise attenuation by these silencers is obtained by fragmentation of the gas flow and reflection of the sound waves upstream.
The main disadvantage of these silencers is that they attenuate the noise over a very limited frequency range, namely the high and medium frequency ranges, leaving unattenuated the low and very low frequencies. In order to get a more significant attenuation of the noise, large overall dimensions of the silencer are necessary, making its construction expensive and limiting its utility. At the same time, in lamellar silencers, cellular silencers and silencers with chambers and screens, the high-velocity and high-temperature gases rapidly degrade the inner elements of the silencer, putting it out of use after a relatively short working period.
There is also known a method of attenuating the noise radiated by the gas jets by means of sound wave diffraction passing through networks provided with Coanda flaps, and by the absorption of the sound waves by the sound-absorbent layers on the surfaces of the flaps. These silencers, although strongly attenuating the noise over a broad frequency range, have the disadvantage of large overall dimensions and excessive complexity.
There are also silencers for attenuating the noise produced by gas jets, which have axially symmetrical outer or inner Coanda nozzles. These devices, technologically and structurally complex, have large overall dimensions and great weight but a limited range of gasodynamic parameters for a stable functioning and induce significant counter-pressures along the stream.